(1) Field of the Invention
The present invention relates to electroluminescent devices, and more particularly to alternating-current powered electroluminescent devices.
(2) Description of the Related Art
Luminescence is a general term that is used to describe the emission of radiation from a solid when it is supplied with some form of energy. The various types of luminescence can be distinguished by the method of excitation that is used to supply the energy. Electroluminescence excitation results from the application of an electric field, which may be either AC or DC. Whatever the form of energy input to the luminescing material, the final stage in the process is an electronic transition between two energy levels. See, e.g. Display Devices, at http/www/geocities.com/Athens/Bridge/2702/CAP4I (Oct. 28, 2002).
Fluorescence occurs when a material emits visible light after being excited by an excitation source applied from outside. A fluorescent lamp, a discharge tube, and a cathode ray tube utilize fluorescence. A material that emits fluorescence is called a phosphor.
Electroluminescence is a solid state phenomenon, which involves the emission of visible or invisible radiation as a result of the absorption of exciting energy. It is a general term which includes both fluorescence and phosphorescence. Invisible light further includes infrared and ultraviolet radiation.
An electroluminescent (EL) display device generally includes a layer of phosphor positioned between two electrodes, with at least one of the electrodes being light-transmissive. At least one dielectric also is positioned between the electrodes so the EL display device functions as a capacitor. When a voltage is applied across the electrodes, the phosphor material is activated and emits light.
Phosphors may be employed in the manufacture of electroluminescent devices. Long-lasting phosphors are known in the art, and include sulfides and oxides. Many long-lasting phosphor products are those with a sulfide as their base crystal, such as ZnS:Cu. Phosphorescence characteristics are influenced by composition, particle diameter, and environment, in particular, the phosphorescence brightness of phosphors.
Other light-emitting materials, such as certain small molecules and certain polymers, may also be employed in the manufacture of electroluminescent devices. Suitable light-emitting small molecules include quinolines, fluorescein, and the like.
Light-emitting polymers (LEPs) may further be employed in the manufacture of electroluminescent devices. Suitable light-emitting polymers include MEHPPV (2-methoxy-5-2′-ethylhexyloxy)-1,4-phenylenevinylene copolymer, MEH-BP-PPV (poly[2-Methoxy-5-(2′-ethylhexyloxy)-1,4-phenylenevinylene-co-4,4′-bisphenylenevinylene]), and MEH-CN-PPV (poly[2-methoxy-5-(2′-ethylhexyloxy)-1-(cyanovinylene)phenylene). These LEPs absorb radiation at about 400 to about 500 nm (blue light) and emit radiation at about 600 and 800 nm (yellow, orange, and red light).
The short lifetime of organic light-emitting polymers (LEPs) has been a major impediment to their use in commercial environments. Organic LEPs are unstable when exposed to air and humidity. In addition to oxygen, other contaminants present in air, such as ozone and NH3, also adversely affect the useful lifetime of LEPs.
Heretofore, lamps fabricated from LEPs have been entirely encapsulated, or have had exposed surfaces coated with protective layers to achieve stability. This large-scale encapsulation/coating process is costly, and requires the use of a relatively expensive transparent material.
Another characteristic of phosphor materials is that the selection of wavelengths of emissive radiation that could be obtained from phosphors that were excitable by a simple electric field was substantially limited to blues, greens and oranges—depending upon the dopant that was used in the phosphor. Radiation of other wavelengths could be obtained from different phosphors, but those phosphors required high-energy photons or an electron beam for excitation. Accordingly, effective provision of electroluminescent radiation having wavelength of a desired spectra—other than blue, green or orange—was difficult to achieve.
It would be useful, therefore, to provide an electroluminescent device capable of emitting radiation at a desired wavelength that was other than blue, green, or orange, but which was powered by an electric field. It would also be useful if the electric field could be supplied by an alternating current source. Furthermore, it would be useful if the electroluminescent device could be produced simply and easily, and without the use of inert atmospheres, high vacuum, sputtering, or the use of electrodes composed of low-work function metals, such as calcium, aluminum, sodium and magnesium, or their oxides.